Will higher Tc superconductors be useful? Fundamental issues from the real world

In this article, we review the reasons why high-temperature cuprate superconductors are inadequate for electric power applications, above liquid nitrogen temperatures, and examine the underlying causes. The most important reason is their low superconducting Cooper pair density, which for thermodynamic reasons reduces the theoretical maximum critical current density. We also discuss how low pair density (and high anisotropy) increase thermodynamic phase fluctuations of the macroscopic quantum pair wave function, which in turn leads to a limitation on the transition temperature itself. Finally, we discuss how, in highly correlated superconductors, there may be a conflict between the conditions necessary to achieve high transition temperatures in the face of phase fluctuations and the conditions necessary to produce strong pairing interactions.